1. Field of Invention
This invention relates generally to wastewater treatment systems, and more particularly to an improved outlet for the extraction of odoriferous and noxious fumes emanating from the contents of a clarifier.
2. Status of Prior Art
Sedimentation is the process by which suspended particles heavier than water are removed by gravitational settling. Sedimentation which comes into play in the natural purification of streams and lakes is also widely used in the treatment of sewage and industrial wastes.
Systems for treating sewage and industrial wastes typically include preliminary settling tanks that collect a good part of the suspended load of impurities before the clarified effluent is discharged into receiving waters or is subjected to further treatment in secondary settling tanks. In the secondary or final settling tanks, there are collected those matters which have been converted into settleable solids or otherwise rendered settleable by biological or related treatment techniques. To improve the efficiency of the system, flocculating or precipitating agents may be added to sewage prior to sedimentation.
A settling tank has four functional zones; namely, an inlet zone, a settling zone, a sludge zone and an outlet zone. For high efficiency, inlets must distribute flow and suspend matter as uniformly as possible within the tank. In the sludge zone, sludge is worked into a sump from which it is withdrawn by gravity or by a pumping action. The main concern of the present invention is with the outlet zone in which the control of outflow is usually effected by a weir attached to one or both sides of an outlet trough or launder which acts as a lateral spillway.
The Anderson U.S. Pat. No., 4,391,704, discloses an outlet zone whose structure and operation is such as to minimize the emission of odoriferous and noxious fumes into the atmosphere. Because incoming waste is saturated with noxious gases and easily volatilized compounds, some degree of odor control is generally necessary in a liquid waste treatment system. In sewage, the objectionable compound which predominates is hydrogen sulfide, a colorless gas having an offensive odor. This gas is highly toxic and a strong irritant to eyes and mucous membranes. Hydrogen sulfide is almost always present in sewage, for it is the product of anaerobic decomposition by bacteria present in the sewage and in the sewage treatment system.
With settling tanks and sewage collection systems having significant detention times, the generation of hydrogen sulfide, particularly in warm weather, can be so vigorous that the gases released from the tank create serious problems. Apart from its unpleasant odor, the presence of hydrogen sulfide in the atmosphere is hazardous to personnel in the vicinity of the tank and may cause permanent injury and even death. And since hydrogen sulfide will condense with water on concrete or metal surfaces to form sulfuric acid, this acid may severely corrode the structures.
Various approaches have heretofore been taken to minimize the adverse effects of hydrogen sulfide. Thus, strong oxidizing agents such as hydrogen peroxide have been added to the sewage system to inhibit the anaerobic bacteria responsible for generating hydrogen sulfide, the agents oxidizing the sulfide to form soluble, non-odoriferous orifice sulfite or sulfate compounds.
Another approach heretofore taken is to cover the settling tanks. But this approach leaves much to be desired; for when the tanks are covered with a concrete or other structure, the reaction of the gases with the structure then gives rise to serious corrosion problems, and the cost of complete covers is very high. Complete coverage of large tanks also creates a large enough volume over the liquid that workers can enter. Therefore, a large volume of air must be blown through this enclosed area to afford a safe working environment for personnel. This large volume of air dilutes the odoriferous compounds and makes them more difficult to treat. Thus, prior attempts to solve the problem of odor control, despite their relatively high cost, have been of limited effectiveness.
The outlet zone disclosed in the Anderson patent functions to localize the odoriferous and noxious fumes to the region of the collection trough or launder from which they are extracted, the settling zone of the tank being uncovered.
The Anderson invention is based on the recognition that almost all of the noxious gases generated in a settling tank are released as the clarified water spills over the weirs mounted on the sides of the trough; hence by capturing these hazardous gases in the trough region and transporting the captured gases to a gas-control system or in otherwise disposing of the gases, one is able to reduce the discharge of gases into the atmosphere to an acceptably safe level.
The Anderson patent provides an outlet zone for a settling. tank in which the trough is covered by a hood that also functions as a scum baffle. In a conventional outlet zone, a baffle which protrudes into the tank water is mounted adjacent to the weir of the trough to prevent grease and other floating matter from being discharged into the trough with the effluent. With the Anderson arrangement there is no need for separate baffle plates; for the hood which acts to confine the gases released in the trough has a baffle integral therewith.
Inasmuch as the hood in Anderson which covers the trough is required to collect a relatively small volume of air as compared to a cover placed over the tank, a significant advantage of this arrangement is that it is not only more effective than a massive tank cover, but is also substantially less expensive to construct and operate.
While the arrangement disclosed in the Anderson patent represents a significant contribution to the art of wastewater treatment, it has certain practical drawbacks. In Anderson, the weir which permits clarified tank water to spill into the trough lies under the gas-confining hood, at least one side of the hood protruding into the water to function as a scum baffle. After prolonged operation, the weir accumulates algae, sludge and other contaminants, and these interfere with the proper operation of the weir. Hence it becomes necessary, on occasion, to remove the entire hood from the trough in order to gain access to the weir so that it can be scrubbed and cleaned, after which the hood must be placed back on the trough. This is a somewhat difficult, costly and time consuming operation.
And if one wishes only to decontaminate the side of the hood which acts as a scum baffle in order to remove scum build-up without, however, removing the hood from the trough, this operation also presents difficulties. The hood in Anderson is molded of fiberglass-reinforced plastic and has a quasiconvex top surface. This rounded surface is not an easy or safe surface for an operator to walk over, either to obtain access to the scum baffle or to inspect the installation, as is sometimes necessary.
Also of prior art background interest are the following U.S. Pat. Nos.:
Imhoff et al.; 1,690,692
Kelly et al.; 2,360,812
Carter; 2,422,394
Wiley; 3,017,951
Maisonneuve et al.; 4,039,458